1. Field of the Invention
The present invention relates to a new class of photoinitiators, especially suitable for low viscous radiation curable compositions that are curable by UV-LEDs and for packaging printing applications.
2. Description of the Related Art
Short run packaging printing is shifting from conventional printing techniques, such as offset printing, to digital printing, where ink jet is one of the preferred technologies. In inkjet printing, tiny drops of ink are projected directly onto an ink-receiver surface without physical contact between the printing device and the ink-receiver. The printing device stores the printing data electronically and controls a print head for ejecting the drops image-wise on an ink-receiver.
Within ink jet for digital packaging printing, there is a clear evolution towards higher image quality and higher printing speeds in combination with LED curing. In order to satisfy these demands, a new print head design is required. These print heads require a specific ink design as they only can operate with very low viscous inks. The inks for high resolution, high speed short run digital packaging printing have to combine low viscosity, low migrating properties after curing and high sensitivity for LED exposure.
Particularly interesting monomers to obtain low viscous ink jet inks have been described in EP 0997508 A (AGFA GEVAERT), which discloses radiation curable monomers containing vinylether and acrylate functions. Suitable monomers and radiation curable compositions having high degrees of conversion and low amounts of volatiles have also been disclosed in WO 2009/053305 (AGFA GRAPHICS).
High sensitivity for UV-LED exposure, preferably 395 nm LED exposure, requires bathochromic photoinitiators. Thioxanthones are known as being particularly preferred photoinitiators for LED exposure. For packaging applications, migration of photoinitiators has to be limited, leading to the need for diffusion hindered thioxanthones. A photoinitiator is considered diffusion hindered when it, for example, contains a polymeric group or at least one polymerizable group, e.g. an (meth)acrylate group.
Several diffusion hindered thioxanthones have been disclosed in the prior art. Polymeric thioxanthones have been disclosed in WO 03/033492 (COATES BROTHERS), WO 2009/060235 (LAMBSON LTD) and EP 1616921 A (AGFA GEVAERT). Polymerisable thioxanthones have been disclosed in EP 2161264 A (AGFA GRAPHICS) and WO 2010/069758 (AGFA GRAPHICS).